Rafael, you're right that your project has to be part of your life, and must intrigue you or be important enough to you to involve your efforts thoroughly over a period of time. The more meaning it has for you, the more meaningful the results are likely to be.
There are certain subjects that have been my projects in the past, and I am still "sensitive" to them when the opportunity comes up. Manikins (the more realistic looking ones) fascinate me in what they say about humankind. Geometric shadow designs where the shadow of an object seems more solid and "real" than the object itself. And I've been documenting instances where I see in use a freeware font I designed years ago (it keeps popping up). Old projects seem to have a way of hanging on.
Several years ago I transcribed all my old 3x5" file cards to the computer; each card representing data for one roll of film shot since the mid-1960's. For complete info I often had to refer to contact sheets, prints, and negative files. In the process of looking over all this old stuff I found that the pictures that really grabbed me were the ones with people, or about people. Those that had a strong human-interest element are more memorable.
So at that point I resolved to concentrate on people pictures. But I admit to some shyness and reservation about poking my camera in others' faces. It seemed easiest to "victimize" non-strangers; those who knew who I am... Like those with whom I had business. In their place of business I was a customer and due polite treatment, further they were stuck and couldn't escape! Perfect way to start.
Clearly my new project was "people at work, or at least doing something." I've pursued that project for several years now, finding RF gear ideal for this. I think now I've mostly "done that" and need to find a way to expand the project or take it to another level. Even though I've gained the confidence in my "gift of gab" to snap strangers too.
So, there's my experience, which may or may not be helpful to you! Good luck...